1. Field of the Invention
The present invention relates to a flue gas cleaning system and in particular to a method and apparatus for minimizing condensation of corrosive agents in the filter compartments of a baghouse installation.
2. Description of the Prior Art
In the typical baghouse installation, flue gases are drawn from a common inlet manifold into a bank of filter compartments wherein the gases are filtered through a plurality of filter bags to remove the suspended particulates from the gases. The filtered gases then exit the compartments and flow into a common outlet manifold from which they are directed into a chemical scrubber to remove the sulfur oxides and other corrosive agents in the gases prior to discharging them into the atmosphere.
In systems of this type, it has been the practice to admit the gases into the filter compartments at a temperature above the dew point temperature of the corrosive agents in the gases while maintaining a relatively uniform gas flow through each of the compartments. This prevents the formation of stagnate gas zones in the filter compartments where the gases could cool down below the dew point of the corrosive agents which would cause the agents to condense in the compartments. For this reason, it has generally been necessary to close off or isolate one or more of the filter compartments when the gas flow rate drops due to a reduced load on the boilers served by the system in order to maintain an adequate flow rate in the remaining compartments. However, when one of the compartments is isolated, the flue gases trapped in the compartment begin to cool and ultimately the corrosive agents condense if the compartment is not put back on line before the temperature of the gases drops below the dew point.
In addition to the foregoing, experience has indicated that even greater accumulations of corrosive agents can be expected to form in the isolated compartments as a result of condensation in the flue gases seeping through the dampers closing off the isolated compartments. For example, in a baghouse installation where each compartment is designed to clean about 80,000 ACFM (actual cubic feet per minute) of the gases, the seepage rate through each of the isolated compartments is in the range of 80-400 ACFM or 0.1-0.5 percent of the gas flowing through the compartment during normal filtering operations. Assuming the flue gases include about 10 parts per million SO.sub.3, which is fairly typical for a coal fired boiler, as the isolated compartment cools the filter bags will be exposed to about 15 gallons of sulfuric acid condensing out of the flue gases trapped in the compartment when it was isolated, and 2-10 gallons of acid for each hour the compartment is kept off line.
Considering the above, it can be seen that acid condensation in the isolated compartments can have a very deleterious effect on the life of the filter bags and the metal housing of the baghouse installation, as well as increasing pressure losses in the gases as they flow through the compartments due to deterioration of the filter bags and the accumulation of sulfates and other particulates on the bags which generally cannot be dislodged by conventional bag cleaning procedures. Moreover, it should be noted that while the filter bags will be subject to acid attack when the compartments are isolated at a temperature below the dew point of the corrosive agents, they will also be exposed when the isolated compartments are brought back on line, or alternatively, when they are taken off line and allowed to cool for routine maintenance, thus further aggrevating the problems associated with acid formation within the compartments.